
where set \(\mathcal {B}\triangleq \lbrace \mathcal {U}_2 \in \mathbb {C}^{M_{i,y} \times M_{i,z} }, t_i\in \mathbb {R}\rbrace \)  with \(\mathcal {U}_2\)  being a 2-way tensor and \(\mathrm {Toep}(\mathcal {U}_2)\)  is a 2-level block Toeplitz matrix, which results from the Vandermonde decomposition lemma for positive semidefinite Toeplitz matrices [1]}.
